Melting Accelerating, Satellites Report, But Data Flow May Cease
by Eli Kintisch, ScienceInsider, March 10, 2011
Credit: Source: Canadian Space Agency
Data published yesterday by scientists at the Jet Propulsion Laboratory (JPL) in Pasadena, California, and colleagues revealed that Earth's ice sheets are melting at a rate that could mean more than 32 centimeters of global sea level rise by 2050. But scientists say their ability to continue to collect and analyze such data is threatened by the commercial sale of the some data, and the possible failure of a key satellite.
The paper, in Geophysical Research Letters, is seen as authoritative because it draws upon and compares two techniques for measuring ice mass. The first is the so-called mass-balance method. It utilizes measurements from satellite and airplane images along with data from computer models to calculate the comings and goings of ice and to produce a total flux. The second is the gravity method, which utilizes NASA's GRACE satellite pair to essentially weigh the ice sheets from space (it measures minute changes in their flight path due to the shifting gravity field of mass below).
The two methods validate each other well, the new paper shows, delivering comparable values for total ice loss, rate of loss, and acceleration of loss. When measuring trends, the GRACE data represent a shorter time series and, therefore, offer a larger margin of error.
But scientists are worried that both tools are in jeopardy. "We're losing our capability to monitor the ice," says Eric Rignot, a scientist with the University of California, Irvine, and JPL and lead author on the study.
An immediate problem is that the Japanese space agency, JAXA, is planning to start charging a much higher cost for the radar scans that researchers rely upon to make the mass-balance measurements. (The Japanese satellite is called ALOS, though the radar image accompanying this story is from a Canadian craft.) The images have been available at a nominal cost, but NASA won't pay the higher rates, says glaciologist Ian Joughin of the University of Washington, Seattle. NASA hopes to negotiate with JAXA to work out a compromise. (Scientists are counting on the fact that there's little commercial interest in the shape of ice sheets to help negotiations.)
"Before NASA could afford it, and now it can't. It's a definite disappointment," says Joughin. Images from a German satellite are filling in, he says, but it provides less coverage than the Japanese sensor. Offering hope isSentinel, a European satellite set to launch in 2012 or 2013, which will have a free data policy. "A 2-year gap is painful, but if [Sentinel] functions the way its supposed to, we'll be OK," says Joughin.
And then there's the uncertain future of gravity measurements. The satellite's designed lifetime ran out in 2007, and Rignot says that while NASA missions have a good track record of outliving their wear-out dates, "there's no approved follow-on mission for GRACE."
GRACE has been a phenomenal success for climate science since its 2002 launch despite skepticism from ice scientists about how well the new technique would work to measure ice sheet masses. Yesterday's paper is confirmation that the mission is delivering unprecedented and reliable data, says Rignot. "I don't think anyone had solid confidence that it could be done well," he recalls. But climatologists would lose a valuable tool if there's no successor to GRACE.
At the continental scale, there is a new assessment of the net mass balance of Antarctica and Greenland. Rignot et al. have updated results, including those from the GRACE gravity measurement satellite, to the end of 2010 and show that the downward trend in ice mass is continuing (stronger in Greenland than in Antarctica). The net rise in sea level associated with this decline is about 1.3 mm/yr, which will likely accelerate with further warming. Complementary analyses of the surface mass balance of Greenland (Tedesco et al., 2011) also show that 2010 was a record year for melt area extent… Extrapolating these melt rates forward to 2050, "the cumulative loss could raise sea level by 15 cm by 2050″ for a total of 32 cm (adding in 8 cm from glacial ice caps and 9 cm from thermal expansion) -- a number very close to the best estimate of Vermeer and Rahmstorf (2009), derived by linking the observed rate of sea level rise to the observed warming.
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